As semiconductor technology progresses, shrinking device dimensions has become an increasingly complex task. Complementing metrology tools, allowing similar improvements in measurement capabilities, are critical for the continual process of this development. Commonly, optical metrology is applied to test structures comprised of a repeating array of identical elements. Optical metrology can acquire highly accurate and precise information on the geometry and material properties characterizing these structures, and thus provide the required information for better process and process control.
Several physical quantities are commonly measured by optical metrology. For example, optical reflectometry measures the reflection intensity for a broad spectrum, over a single (or small set) of incidence directions and different polarizations. Ellipsometry allows, in addition, access to information on the relative phase between different polarization states. Dome scatterometry measures the sample reflectivity over a large range of incidence directions, for a small set of wavelengths. In addition, different interferometry techniques are used to measure the phases of the scattered optical components. These could be the spectral components or the angular ones.
Various metrology techniques have been developed allowing measurements of the parameters of complex structure (three-dimensional structure) such as semiconductor wafers. Examples of such techniques are described in US 2013/308131, U.S. Pat. No. 8,289,515, US 2013/282343, all assigned to the assignee of the present application.